In treating stenosis in coronary arteries, a restenosis occurs in 30-60% of the cases. It is known that a treatment with beta- or gamma- (x-ray) radiation will decrease the occurrence of restenosis substantially. Methods to apply this radiation to the treated stenosis are presently subject to intensive research.
Another example of an application of the present invention is treatment of cancer tumours where it is desired to deliver radiation locally.
The use of radioactive pellets or balloons etc. to introduce radioactive isotopes is known. The radioactive isotopes are introduced via a catheter, a needle or similar to the treated area. Such methods have some drawbacks, such as limited half-life of the isotope as well as the fact that the devices used emit radiation continuously. Such devices sometimes require substantial efforts to control radiation in the environment outside the patient and also exhibit problems with dose control.
The importance of controlling the radiation distribution along the vessel and of ensuring that only tissue that has been treated by coronary angioplasty will receive radiation while as little as possible radiation is applied to healthy tissue has been recognised. Thus, Novoste Corp. has introduced an array of isotope elements, enabling the radiated length of the vessel to be predetermined stepwise.
One known way to overcome some of the above drawbacks is the use of a miniature electrical x-ray tube including a cold cathode. Such a tube may be switched on and off due to its electrical activation. An example of such an x-ray tube is described in the U.S. Pat. No. 5,854,822.
However, the conventional miniature electrical x-ray tubes exhibit a problem in that the delivery of radiation has a very limited spatial extension. These radiation sources can in essence be regarded as approximately "point like" radiation sources.
Another problem present with a conventional miniature electrical x-ray tube is the dissipated heat. The temperature increase with respect to the body temperature should not be high enough to produce a local temperature exceeding approximately 41.degree. C.
One way to handle the dissipated heat is to provide cooling by flushing a saline solution onto the tube. Cooling by flushing exhibits the problem of enlarging the geometry since the saline must be delivered to the source of heat and therefore must be directed by some means that inevitably will occupy space. Also, a flow through a catheter has to be established and maintained to cool the x-ray tube which is generally awkward.
Another way to reduce the dissipated heat is the use of a pulsed source wherein the electrically activated tube is turned on intermittently. However, a pulsed source exhibits the drawback that the treatment time will be prolonged correspondingly, since the received dose must be held constant. This is costly and increases the discomfort for the patient.
Yet another way to reduce the dissipated heat is to apply a sufficiently low current to the conventional x-ray tube. In consequence, the treatment time has to be correspondingly increased in order to apply the appropriate dose of x-ray radiation. This, of course, is disadvantageous in that the longer treatment time is inconvenient to the patient and calls for raised costs in the hospital.
Yet another problem experienced with the conventional techniques including miniature electrical x-ray tubes is the erosion of the electrode material. As the target is bombarded by high-energy particles, the impacts will tear away atoms from the surface. If these atoms are ionized they may be transported away from the target to be deposited on the cathode or on other parts of the interior of the x-ray tube.
Therefore, there is a need for an improved miniature electrical x-ray tube.